Augers primarily are used for general exploration, advancing and cleaning the borehole, and drilling accessible borings. Augers also are used for various construction operations, such as drilling drainage wells and excavating for piers and caissons. Disturbed or undisturbed samples can be obtained from boreholes advanced by augering methods. However, disturbed samples may not be representative of the in situ deposit because materials may have segregated during the augering process or may have been contaminated with soils from different depths. The quality of undisturbed samples also may be questionable as a result of stress relief, especially if drilling mud is not used to stabilize the borehole. Augers cannot be used for soils in which the gravel particles or rock fragments are greater than approximately one-tenth of the diameter of the hole.
Disk augers and solid- and hollow-stem flight augers are helical augers. Spoon augers are similar to closed-spiral augers. The diameter of power augers ranges from approximately 2 inches to 96 inches. The depth of samples obtained with power equipment can exceed 100 feet or more, depending upon the ground water conditions and the type of equipment that is used.
In general, power augers can be used wherever the borehole is stable and will remain open. The principal disadvantage of sampling by auger methods is that samples are highly disturbed and soils from different strata can be mixed. Because of the potential for mixing soils from different strata, stratigraphic logging using cuttings from auger borings is extremely difficult. An exception does exist, however. When a hollow-stem auger is used, the center plug can be removed at any time and either disturbed or undisturbed samples can be obtained with conventional sampling equipment. Large bucket augers also can be used for drilling large-diameter boreholes that will permit a person to enter and obtain hand-carved samples. The limiting depth for power-augering usually is controlled by the power that is required to rotate the auger or the depth to the ground water table. For continuous-flight augers and bucket augers, the limiting depth is about 100 feet. For short-flight augers, the depth is limited to the length of the kelly on the drill rig, which is about 10 feet to 20 feet, depending on the particular device.
The bucket auger is an open-top metal cylinder with one or more slots in its bottom that permit soil to enter as the bucket is rotated and downward pressure is applied. The slots are reinforced and usually are equipped with teeth or a cutting edge. To operate, the bucket auger is attached to the kelly rod. It is driven by a rotary table. Rotation and downward pressure are used to fill the bucket. When the bucket is full, the rotation is stopped and the bucket is lifted from the borehole. When the bucket is clear of the borehole, it can be emptied by tipping. Some buckets, such as the Vicksburg hinged auger, are equipped with hinges and a trip release, which allow the bucket to be opened for dumping. The principal advantage of the bucket auger is the rapid excavation of small- or large-diameter holes to relatively great depths. The principal disadvantage is that most bucket augers cannot be used for drilling cohesionless materials below the water table or to sample gravelly soils.
The flight auger is the most commonly used power auger. It consists of one or more flights of helical or spiral fluting attached to a torque bar. Hence, the respective auger is called single-flight or continuous-flight. Likewise, the torque bar may be solid or hollow, which explains the terms “solid-stem” and “hollow-stem.” One end of the torque bar is connected to the drill, and the other end can be fitted with a pilot bit and cutting teeth or some other type of bit for ripping the material to be drilled. The spiral fluting acts as a platform for removal of cuttings to the surface.
The diameters of solid-stem flight augers range from 21⁄4 inches to 48 inches or larger, although flight augers with diameters to 12 inches are the most common. The principal advantage of solid-stem flight augers is that a minimum number of tools are required to advance the borehole. These augers can be used for drilling in stable soils, including gravel and soft rock. They do not work well for drilling in hard, cemented materials. Solid-stem flight augers cannot be used for drilling cohesionless materials below the water table because the material tends to wash off the auger flights and the holes generally will not remain open.
The hollow-stem auger consists of a section of seamless tube that is wrapped with spiral flight. It is fitted with an adapter cap at its top and a center plug and cutter head at its lower end. The cutter head is connected to the drill rig by drill rods, which attach to the adapter at the top of the auger. The cutter head may be equipped with finger-type bits for general drilling, fishtail bits for drilling cohesive materials, or carbide teeth for drilling in hard or stiff deposits. The adapter cap is designed to hold the center plug in place as the auger is advanced. It ensures that the center stem and bit rotate with the auger. When drilling and sampling with the hollow-stem auger, the hole usually is advanced with the center plug and stem in place, although the center plug may be omitted for certain soils. The hollow-stem flights and center stem can be added as necessary. At the desired sampling depth, the center stem and plug can be removed and sampling may be conducted through the hollow stem of the auger. The hollow stem serves as casing.
Two types of connectors are used to connect stems of continuous-flight augers. Screwed joints are easy to connect and form a watertight, rigid, stiff connection. The disadvantages are that the auger cannot be operated in reverse, and the stems may be difficult to disconnect, especially if soil particles become wedged in the threads or the threads become worn or damaged. Splined joints transfer torque between auger stems by an octagonal socket and shank jaw coupling or a straight-keyed coupling. Tension is transferred by a removable threaded setscrew or pin.
Splined joints are fairly easy to connect and disconnect, although they may be somewhat difficult to align during assembly. They also can transfer a reverse torque between the auger stems. The principal disadvantages are that the joints are not watertight and must be cleaned regularly before assembly. If O-ring seals are used to affect a watertight seal, the O-rings must be replaced frequently because of wear. The hollow-stem auger can be used in loose, cohesionless deposits below the ground water table. The inside diameter of hollow-stem augers ranges from about 23⁄4 inches to 12 inches. The principal advantage of hollow-stem, continuous-flight augers is that the auger serves as a casing for sampling soft or unstable soils. Furthermore, it is likely that less disturbance to the formation is caused by augering than by driving casing. The principal disadvantage of the hollow-stem auger is the cost and size of the equipment that is required to operate the auger.